1. Field of the Invention
The invention relates to an electrophotographic photoconductor. In particular, it relates to an electrophotographic photoconductor comprising a conductive support, undercoating layer and photosensitive layer.
2. Description of the Related Art
An electrophotographic process using a photoconductor comprises the steps of placing the photoconductor in the dark and charging the surface thereof evenly by corona discharge, exposing a region to selectively discharge electric charges and form an electrostatic image in the nonexposed region, and subsequently depositing the colored charged particles (toner) to a latent image by electrostatic attraction and the like to visualizing it, thereby forming an image.
Primary characteristics required of photoconductors in the above serial process are as follows.
(1) It can be charged evenly to a suitable potential in a dark place. PA1 (2) It has a high chargeability in a dark place and electric charges are less discharged. PA1 (3) It has an excellent photosensitivity and discharges electric charges immediately by exposure.
Further, photoconductors needed to have stability and durability, for example, little residual potential because of easy discharge of the surface of the photoconductor; excellent mechanical strength and flexibility; stable electric properties with no change of chargeability, photosensitivity, residual potential and the like even after repeated use; and endurance against heat, light, temperature, humidity, ozone deterioration and the like.
Electrophotographic photoconductors are currently used for practical purposes. Such photoconductors are prone to generate carrier implantation from the surface of the conductive support, so that image defects are produced because of disappearance of or decrease in surface charges form a microscopic view. In order to solve the problem, and further to coat defects of the surface, to improve the charging properties and to improve adhesive and coating properties of the photosensitive layer, an undercoating layer is provided between the conductive support and photosensitive layer.
Conventional undercoating layers contain various type of resin materials and those containing titanium oxide powder or the like. Known materials for the undercoating layers formed of a single layer include resin materials such as polyethylene, polypropylene, polystyrene, acryl resins, vinyl chloride resins, vinyl acetate resins, polyurethane resins, epoxy resins, polyester resins, melamine resins, silicon resins, polyvinyl buthyral resins, polyamide resins; and copolymer having more than two repeating units of these resins; casein, gelatin, polyvinyl alcohol, ethyl cellulose and the like. Among them, polyamide resin is preferable (disclosed in Japanese Unexamined Patent Publication Sho 51 (1976)-114132 and Japanese Unexamined Patent Publication Sho 52 (1977)-25638). However, the electrophotographic photoconductors having a single layer formed of polyamide etc. as an undercoating layer have a defect of great residual potential storage, which reduces sensitivity and induces an overlap of an image. This tendency becomes conspicuous under a low humidity.
Therefore, for preventing the image defect and improving residual potential, Japanese Unexamined Patent Publication Sho 56 (1981)-52757 discloses an undercoating layer containing surface-untreated titanium oxide. In addition, Japanese Unexamined Patent Publication Sho 59 (1984)-93453 and Japanese Unexamined Patent Publication Hei 2 (1990)-81158 disclose an undercoating layer containing in the surface titanium oxide particles coated with alumina and the like for improving dispersion of the titanium oxide powder. Further, Japanese Unexamined Patent Publication Sho 63 (1988)-234261 and Japanese Unexamined Patent Publication Sho 63 (1988)-298251 propose an undercoating layer comprising titanium oxide particles and binder resin in which the mixing ratio of titanium oxide is optimized for prolongation of the life of photoconductors.
In the above described undercoating layer containing titanium oxide powder, titanium oxide having a grain-like shape has been used.
Coating methods used for forming the electrophotographic photoconductor include a spray method, bar coat method, roll coat method, blade method, ring method, dip coating method and the like. According to the dip coating method shown in FIG. 1, the electrophotographic photoconductor is formed by immersing a conductive support in a coating tank filled with a coating solution for the photosensitive layer and pulling up the immersed conductive support at a constant or changing speed. The dip coating method is often used for forming an electrophotographic photoconductor because it is relatively simple and excellent in productivity and cost.
Preferably, resins used for the undercoating layer are hardly soluble in a solvent of the coating solution for the photosensitive layer. Generally, either alcohol soluble or water soluble resin is used. The undercoating layer is formed by preparing an alcohol solution or dispersed solution of the resign as a coating solution for the undercoating layer and by coating the support with the coating solution for the undercoating solution.
When the undercoating layer comprises titanium oxide powder and binder resin in which the ratio of titanium oxide is small as compared with the binder resin, the volume resistance of the undercoating layer increases and carriers transportation generated by exposure are controlled or prevented. As a result, the residual potential raises, thereby forming an overlap in an image. Furthermore, when electrophotographic photoconductors are used repeatedly, they are significantly affected by the accumulation of residual potential, temperature and humidity. In particular, the accumulation of residual potential becomes conspicuous at a low humidity, thereby degrading stability and failing to provide sufficient properties of the phoroconductor.
With increase in the content of titanium oxide, these problems are solved. But, if the electrophotographic photoconductor is repeatedly used, the residual potential tends to be stored. Especially, the tenancy is significantly revealed at a low humidity, failing to completely solving the problem of the stability in a long duration and environmental properties.
Moreover, if the titanium oxide content increases to a ratio at which the content of the binder resin becomes virtually zero, the film strength of the undercoating layer decreases and adhesiveness between the undercoating layer and the conductive support is weakened with the result that after repeated use of the photoconductors the photosensitivity thereof is degraded due to the breakage of the film and the image is adversely affected. Additionally, photoconductors have a drawback of an abrupt decrease in volume resistance and low chargeability.
The titanium oxide powder used for the undercoating layer of the conventional invention has a particle size of 0.01 .mu.m or more and 1 .mu.m or less in the observation of the microscope, and the mean of the aspect ratio thereof is in the range of 1 or more to 1.3 or less. The particles have approximately spherical shape (hereinafter referred to "grain-like shape") despite some degree of unevenness. When the titanium oxide dispersed in the undercoating layer has the grain-like shape, the particles come into contact with each other at a point and the contact area thereof is small. Therefore, unless the content of the titanium oxide exceeds a certain level, the resistance of the undercoating layer is significantly high and the photoconductor properties, especially sensitivity and residual potential, are degraded. Accordingly, in case of titanium oxide of the grain-like shape, a larger content of titanium oxide is required in the undercoating layer.
Despite the improvement in the properties with the larger ratio of titanium oxide content, the photoconductor will never fail to be deteriorated through repeated use over a long time because of a weak contact between the particles.
When the content of titanium oxide is increased, the dispersion of titanium oxide to binder resin, in addition, dispersion and stability of the coating solution for the undercoating layer are deteriorated. This produces coating unevenness when the undercoating layer is applied in the process of forming the photoconductor, thereby failing to provide excellent image properties. Therefore, a coating solution for the undercoating layer which satisfies a sufficient dispersion and stability has been demanded.